[TenTec] [] Running a Century 21 on external 12V power

[Byron Cordes]

The print I looked at had the 4.7 ohm and a 240 uf cap to filter the 12 volt supply to the audio amp. Normally the resistor would open up but it's a chance. A choke would be a better choice but cost would be much higher than a resistor.  Try jumping a cap across the 12  volt to ground on the audio board. Check grounds on audio board  bad solder joint?

Sent from my iPad

> On Jun 5, 2017, at 7:15 PM, Josh Gibbs <gibbsjj at gmail.com> wrote:
> 
> Thanks Byron. Grounding to PS ground had no effect.
> 
> I don't see a 4.7 Ohm resistor anywhere... are we talking a resistor on the
> audio board? Here is the schematic for the audio board:
> 
> https://drive.google.com/file/d/0B_pjAwEfeYcdUk9JWDROazFLVkE/
> view?usp=sharing
> 
> All the schematics for the boards are here:
> 
> https://drive.google.com/open?id=0B_pjAwEfeYcdWVoxTWlMbU9xODg
> 
> 
> -Josh
> 
>> On Mon, Jun 5, 2017 at 2:02 PM, Byron Cordes <byroncordes at icloud.com> wrote:
>> 
>> Try grounding ground side of speaker to ground from power supply.
>> If hum is still there there is a 4.7 ohm resistor that might be shorted
>> Or a 270 uf cap open  both feed the audio amp
>> 
>> Byron Cordes
>> 
>>> On Jun 5, 2017, at 3:42 PM, Josh Gibbs <gibbsjj at gmail.com> wrote:
>>> 
>>> Hi Chris,
>>> 
>>> The hum is present in the speaker, just not so noticeable because it is
>>> much less sensitive and much further from my ear. I suspect the speaker
>> is
>>> less responsive to frequencies below 120Hz than my headphones, which
>> helps.
>>> 
>>> I have tried other headphones. I have also listened to the audio directly
>>> at the output of the audio amp board. Even when I short the input to the
>>> audio board to ground, I can still hear the hum. Even when I powered the
>>> pre-amp and audio amp boards via external 12VDC the hum remained. I'm
>>> pretty confident the hum is magnetically coupled. The only time I don't
>>> hear the hum is when I power the radio entirely from external 12 VDC.
>>> 
>>> Is this normal for this rig, or is there a problem? That is what I'm now
>>> attempting to determine!
>>> 
>>> The radio works great otherwise, and I likely would never have noticed
>>> anything if I hadn't plugged in my headphones =)
>>> 
>>> I've learned a lot while working on this radio, which was one of my goals
>>> in buying it. Big thanks to the the awesome amount of knowledge of the
>>> Ten-Tec list, and willingness to share it!
>>> 
>>> 73,
>>> 
>>> -Josh WA7FPV
>>> 
>>>> On Mon, Jun 5, 2017 at 12:29 PM, Chris Howard w0ep <w0ep at w0ep.us>
>> wrote:
>>>> 
>>>> 
>>>> 
>>>> 
>>>> I'm not an electronics whiz.  But to me, if you're getting
>>>> the noise in headphones but not in the speaker, it sounds
>>>> like the headphone cord is picking it up.
>>>> 
>>>> If you have cracked capacitors in the power supply,
>>>> I wonder if you've got a 120 Hz spark gap going there.
>>>> The headphone cord is acting as an antenna and picking it up.
>>>> 
>>>> How to test that theory, turn down the audio on the Century 21.
>>>> Use the same headphones on some other piece of electronic
>>>> equipment in close proximity to the C-21 and see if you get the noise.
>>>> 
>>>> 
>>> _______________________________________________
>>> TenTec mailing list
>>> TenTec at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/tentec
>> _______________________________________________
>> TenTec mailing list
>> TenTec at contesting.com
>> http://lists.contesting.com/mailman/listinfo/tentec
>> 
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec